The invention relates to a rotary disc filter element having a large surface area for tank filters, such filter element having a hub, a circular bottom plate mounted thereon, a filter mesh clamped above it, and a supporting mesh arranged between the bottom plate and the filter mesh.
Rotary disc filter elements having a large surface area filtering means in compression-loaded tank filters are described in U.S. Pat. No. 4,201,670 or the equivalent German Offenlegungsschrift No. 27 05 046. The individual rotary disc filter elements are arranged on a central shaft and clamped firmly together so that they are sealed. All the rotary disc filter elements together form a filter unit. The purpose of rotary disc filter elements is to eliminate impurities from unfiltered material and to draw off the clean filtrate. The distance between the rotary disc filter elements is determined by the height of the tank.
The deposited filtration residues settle on the upper filter mesh of the rotary disc filter elements and form the filter cake. As the total filtration time increases the filter cake grows and becomes thicker. The height of the total filter cake which is formed is determined by the distance between two rotary disc filter elements standing one above the other. In order to remove the filter cake from the disc filter elements the filter unit is set in rotation. The filter cake is removed from the rotary disc filter elements by the centrifugal force.
The filter cake has a very considerable weight. It rests on the upper surface of each rotary disc filter element. In the past, in order to prevent twisting, bowing or bending of the individual rotary disc filter elements under the load of the filter cake the rotary disc filter elements have been supported on the underlying filter element by means of supporting feet. These supporting feet are arranged evenly spaced over the whole outer periphery of each rotary disc filter element.
It is, however, a disadvantage of this method of support that when material to be filtered is applied to the filter mesh and the filter unit is simultaneously rotated slowly these feet cause turbulence to be set up in the region of each supporting foot so that washing off occurs or no primary flooding occurs on the filter mesh in the region of the turbulence. When the actual filtration begins poor filtrate or even unfiltered material enters the filtrate at these points and contaminates it. A constant, effective free filter surface and thus an even hourly output and total capacity are only assured from the beginning over the whole duration of the filtration charge by an even build-up of the filter cake. Washed-off or clean points quickly block the mesh and thus reduce the free effective filter surface.
A further, disadvantage of the supporting feet occurs when the disc filter elements are being cleaned. The supporting feet impede the filter cake being thrown off by the centrifugal force and prevent the pieces of filter cake from slipping off freely. More or less large pieces are retained by the supporting feet. This results in an imbalance in the rotating filter unit and consequently not inconsiderable damage to the machine and the structure. With more highly viscous filter cakes it can even happen that the supporting feet are bent outwards or completely broken off. It is not rare, despite a long period of rotation, for the filter elements not to be one hundred percent cleaned but residues of filter cake are still located in the region of the feet.